Power semiconductor devices have many industrial applications, such as power amplifiers, power converters, low noise amplifiers and digital Integrated Circuits (IC) to name a few. Some examples of power semiconductor devices are Schottky diode, Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET), Insulated Gate Bipolar Transistor (IGBT) and double diffused Metal-Oxide-Semiconductor Transistor (DMOS). The termination structure of power semiconductor devices often requires a high quality semiconductor oxide layer such as silicon oxide. For medium to high voltage devices, a high quality semiconductor oxide layer that is both deep and wide (for example, of the order of ten microns) is often required to insure a high breakdown voltage Vbk and low leakage current Ilk. While semiconductor oxide layers of thickness around 1 micron can be thermally formed or deposited, it can take more than two hours process time already just to form a 0.5 micron thick thermal oxide. Besides being of lower quality, a deposited oxide thickness of a few microns is already considered quite thick in that its dielectric property non-uniformity can be a problem. Manufacturing issues with forming a deep and wide oxide filled trench include: processing time, non-uniformity, and high stress levels.
FIG. 4 illustrates a prior art termination structure disclosed in U.S. Pat. No. 6,309,929 entitled “METHOD OF FORMING TRENCH MOS DEVICE AND TERMINATION STRUCTURE” by Hsu et al and granted on Oct. 30, 2001. The simultaneously fabricated trench MOS device, located to the left of the termination structure, is not shown here to avoid unnecessary obscuring details. The termination structure includes a number of Schottky diodes formed between an anode electrode 160A (delimited by a photoresist pattern 165) and first semiconductor layer 100A at numerous mesa surface locations 115A. The Schottky diodes are, through the anode electrode 160A, electrically connected to a number of MOS structures made of first conductive layer 140, gate oxide layer 125 and the first semiconductor layer 100A. The MOS structures include a poly spacer 122 bordering an important dielectric layer 150 for sustaining a high voltage drop between second substrate 100B and the anode electrode 160A. In an embodiment, the dielectric layer 150 is a termination oxide directly formed upon a trench. Either thermally grown or deposited, the so formed termination oxide thickness is limited to only about 0.2 micron to about several microns due to manufacturing equipment tooling and silicon-oxide interface stress limitations. So the formation of deep and wide high quality semiconductor oxide layer under this prior art can be an issue.
In a paper entitled “Feasibility study of a junction termination using deep trench isolation technique for the realization of DT-SJMOSFETs” published in Proceedings of the 20th International Symposium on Power Semiconductor Devices & IC's, May 18-22, 2008, Orlando, Fla. by Mahfoz-Kotb et al, an alternative dielectric material called BenzoCycloButene (BCB) was tried in lieu of semiconductor oxide for deep trench fill. Besides complicating the associated device fabrication process with an added new material (BCB), the resulting dielectric material quality was found to be inferior compared to thermal oxide. Thus, there is an ongoing need of fabricating high quality trench semiconductor oxide layer that is both deep and wide without incurring a high thermal budget and with good production throughput.